Because of the complex nature (heterogeneity, small porosity, and nano-Darcy permeability) of rocks in unconventional reservoirs, traditional methods of measuring porosity and saturations do not work. Thus starting in the early 1980's, a number of methods were tested and proposed for evaluation of unconventional reservoirs. The two more common methods are: thermal extraction of pore water and hydrocarbons (Retort), and solvent extraction of pore water and hydrocarbons (Dean Stark). The gas-filled volume is measured by pycnometry and following Boyle's law of gasses. By adding the measured volumes of the extracted water, oil, and gas, the pore volume in the rock may be obtained, and therefore the total porosity (Φ). The fraction of this porosity filled with gas, water, and oil are the gas, water, and oil pore fluid saturations (Sg, Sw, So).
A problem arises because the volumes of oil that are measured by Retort or inferred (by weight loss) by Dean Stark do not coincide. This leads to differences in the calculated Φ, Sg, Sw, So, and thus in the characterization of the reservoir properties.
There are multiple publications on reservoir characterization and the evaluation of unconventional reservoir properties of porosity and pore fluid saturations using solvent extraction and thermal extraction of pore fluids. However, these methods are treated as independent and exclusive of each other.
Geochemical analysis of organic richness and maturation of source rocks are commonly conducted using pyrolysis (Rock-Eval, Rock-Eval 6, HAWK, or equivalent), using a programmed temperature heating of the samples, in an inert atmosphere, to define the free hydrocarbons present in the sample and the products associated with the cracking of heavy hydrocarbon components, including kerogen. This method, common to the geochemistry community, has not been previously integrated as a supplementary method for evaluating pore volumes and saturations or as an equivalent method to Retort analysis. For convenience in this description, we refer to geochemical pyrolysis as Rock-Eval, but geochemical pyrolysis may alternatively include equivalent systems and methods for geochemical pyrolysis.
A literature search of relevant articles outlining the three methods is provided below. Some references were written with the specific objective to compare and contrast the thermal extraction method versus the solvent extraction method. Please notice however that, even in these references, the two methods are treated as independent from each other, not supplementary to each other. Rock-Eval is typically not mentioned and is considered as a separate evaluation used for geochemical evaluation.
Luffel, D. L. and Guidry, F. K., 1989. Reservoir Rock properties of Devonian Shale from Core and Log Analysis. SCA International Symposium on Core Analysis. Dallas, Texas, vol. 1, no. 8910.
Spears, R. W. Dudus, D. Foulds, A. Passey Q. Sinha, and W. L. Esch 2011. Shale Gas Core Analysis: Strategies for Normalizing between Laboratories and a Clear Need for Standard Materials, presented at the SPWLA 52nd Annual Logging Symposium, 14-18 May, Colorado Springs, Colo.
Handwerger, D. A., Suarez-Rivera R., Vaughn, K. I. et al. 2011. “Improved Petrophysical Core Measurements on Tight Shale Reservoirs Using Retort and Crushed Samples. Paper SPE147456 presented at SPE Annual Technical Conference and Exhibition, Denver, Colo., 30 October-2 November.
D. A. Handwerger; D. Willberg; M. Pagels; B. Rowland, and J. F. Keller. 2012. Reconciling Retort versus Dean Stark Measurements on Tight Shales. SPE 159976 presented at the SPE Annual Technical Conference and Exhibition held in San Antonio, Tex., USA, 8-10 Oct. 2012.
Pallatt, N. and D. Thornley 1990. The role of bound water and capillary water in the evaluation of porosity in reservoir rocks. Symposium on Core Analysis. London, U.K., vol. 1, no. 90012, 15 p.
D. A. Handwerger, J. Keller, K. Vaughn, 2011. Improved Petrophysical Core Measurements on Tight Shale Reservoirs Using Retort and Crushed Samples. SPE-147456-MS presented at the SPE Annual Technical Conference and Exhibition, 30 October-2 November, Denver, Colo., USA.
T. Jiang, E. Rylander, P. M. Singer, R. E. Lewis, S. M. Sinclair. Integrated Petrophysical Interpretation of Eagle Ford Shale with 1-D and 2-D Nuclear Magnetic Resonance (NMR). 2013. SPWLA-2013-LL presented SPWLA 54th Annual Logging Symposium, 22-26 June, New Orleans, La.
M. Labus, K. Labus and P. Bujok. 2015. Thermal Methods Implementation in Porosity Determination of Shale Rocks. Presented at the 77th EAGE Conference and Exhibition 2015. (also Determination of the pore space parameters in microporous rocks by means of thermal methods. Journal of Petroleum Science and Engineering. Volume 127, March 2015, Pages 482-489).
Konoshonkin, D V; Parnachev, S V, 2015. Existing approaches to tight rock laboratory petrophysics: a critical review. IOP Conference Series: Earth and Environmental Science, Volume 24, Number 1, 2015, pp. 12042-12052(11).